The tasty brews we enjoy are, for the most part, created using the same four ingredients: hops, malt, water and yeast. These ingredients all have their own unique flavors and aromas that are most of the time simple to identify. They do, however, lend off flavors and aromas if the beer or wort are not handled properly. The most common undesirable traits are diacetyl, dimethyl sulfide (DMS), oxidation, sourness and skunkiness. These flavors and aromas can be prevented by knowing where in the process they occur and how to avoid allowing them to form. All of the off flavors listed, with the exception of DMS, occur in the final stage of our beer making process.Identifying Off Flavors and FixesDiacetylDiacetyl is one of the compounds formed by yeast during fermentation. It has a buttered popcorn or butterscotch flavor and aroma. Most often it’s found in beers that have been rushed or lagers that have not been given a chance to rest. If the yeast is given enough time at the right temperature the diacetyl will be reabsorbed. With lagers, a diacetyl rest is used to clean up the flavor of the beer. Once fermentation is close to complete, you want to bring a lager up to ale temperature (65-72 F) to allow the yeast to finish working. This rest occurs naturally with ales as long as they are allowed to finish fermentation without being rushed. A good rule for most ales is to allow them to ferment at least 10 days.DMSDimethyl sulfide is a compound that is naturally occurring in the malt that we use to make our homebrews. It can be characterized by the aroma or flavor of creamed corn or cooked vegetables. One of the reasons we boil is to evaporate this compound. When boiling you want the wort to be completely uncovered so that the compounds don’t condensate on a lid and end up right back in the wort. Chilling the wort as fast as possible is also a very important step because the compound is formed between 120-200 F.OxidationOxidation occurs when the finished beer is exposed to a large amount of air. It will lend to a cardboard or flat flavor and can smell almost like sherry. In older beers, this is sometimes a desirable flavor. In younger brews, however, it means that either the beer was transferred with too much exposure to the air or that it sat too long in the fermenter. Once fermentation is complete, the beer needs to be transferred to an air-tight, sealable container, like a bottle or keg, that carbon dioxide can be added to in order to purge the air. During the transfer, you want to fill the container from the bottom up by using a siphon.

SournessSometimes a sour beer can be a desirable thing, but when it’s unintended it can be a terrible experience. Sour flavors and aromas can mean that during the brewing process something that touched the wort or beer was not properly sanitized. After the boil is complete, everything that touches our brew needs to be thoroughly sanitized. This will ensure that no wild yeast or bacteria get into our brews, potentially ruining them.SkunkinessWe all have had a skunky beer. Though it has become a common flavor for some brands, it is never a good thing. The skunky flavor is created when ultraviolet light hits the beer or wort once the boil is over. The light breaks down compounds in the hops that then become a skunky flavor rather than the wonderful piney-citrus notes we all love in a good IPA. Make sure that any clear fermentors are covered and hidden from sunlight and use bottles that are not green or clear. These measures will help prevent your homebrews from becoming skunky.All off flavors are avoidable. As long as we pay attention to the causes, we can ensure that none of our brews end up being dumped down the drain.